Method and a device for improved retransmissions

ABSTRACT

The invention discloses a method for use in a cellular system with an RBS which controls traffic to and from a cell, with one User Equipment, UE, in the cell. The RBS and the UE receive and transmit information between each other, one of them being a sending party and the other one is a receiving party, and errors in messages which are received can be detected by the receiving party by a first error detection method and if errors are detected in received messages, the receiving party can request a retransmission from the transmitting party. The receiving party stores the order in which retransmissions are requested, so that erroneous retransmissions may be detected by the receiving party without the first error detection method.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/526,156, filed Aug. 6, 2009, entitled “Method and a Device forImproved Retransmissions,” which is a national stage entry under 35U.S.C. §371 of international application no. PCT/SE2007/050077, filedFeb. 7, 2007, entitled “A Method and a Device for ImprovedRetransmissions”. Each of the '156 and '077 applications areincorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention discloses a method for use in a cellular accesssystem in which there can be at least one Radio Base Station whichcontrols the traffic to and from a cell in the system, and in whichsystem there can be at least one user with a User Equipment, UE, in thecell.

The RBS and the UE can receive and transmit information to and from eachother, so that one of them is a sending party and the other one is areceiving party, and errors in messages which are received can bedetected by the receiving party by means of an error detection method.If errors are detected in received messages, the receiving party canrequest the transmitting party to retransmit a message in which an errorhas been detected.

BACKGROUND

In a cellular access system such as a cellular telephony system, thereis at least one cell, with one or more users in the cell, each of whichuser has a User Equipment, a UE, by means of which the user cancommunicate with the system. The system will also comprise at least oneRadio Base Station, an RBS, which, inter alia, serves to control trafficto and from the UEs in the cell.

Thus, the UE and the RBS will communicate with each other, with the UEtransmitting information to the RBS and receiving information from it,and likewise for the RBS with regard to the UE. In order to ensure thatmessages which are sent by one of the parties (UE/RBS) are receivedcorrectly by the receiving party (RBS/UE), a number of techniques areemployed.

If the receiving party detects an error in a message that has beenreceived from the transmitting party, or that a message has not beenreceived at all, the receiving party can request a retransmission by thetransmitting party, usually in the form of a NACK-message, NegativeAcknowledge. In order to minimize the transmission delay it is essentialthat the receiving party can detect a transmission error as soon aspossible. It is especially important to detect errors on retransmittedradio blocks or NACK-messages as soon as possible, since they correspondto already delayed radio blocks. A number of methods are used in orderto achieve this, i.e. to detect erroneous retransmissions or NACKmessage errors:

One way to detect all kinds of transmission errors is to use a wellprotected header with a sequence number followed by separately codeddata protected by an error detection code. If the header can be decoded,but the data is detected erroneous, the receiver can request aretransmission, i.e. send a NACK, for the block with the given sequencenumber.

It is also possible to use a timer for each retransmission, so that thereceiving party starts a timer each time it requests a retransmission,i.e. sends a NACK. If the timer expires before the retransmissionarrives, the receiver can conclude that either the NACK or theretransmission was erroneous, and a new NACK is sent.

However, with each of the methods described above, there are drawbacks,as illustrated in the following:

The method of detecting errors through error detecting codes requiresthe header to be separately decoded and correctly received, so that thesequence number of the lost packet is known. If the sequence number isnot known, other methods are needed.

The method of using a timer for retransmissions is quite expensive,since all timers need to be maintained and controlled. In addition, itis difficult to obtain the correct value of a timer, sinceretransmissions may be delayed not only due to transmission errors, butalso because of multi-user scheduling.

SUMMARY

Thus, as described above, there is a need for a solution by means ofwhich erroneous retransmissions and erroneous NACK messages between a UEand an RBS in a cellular access system can be detected by the receivingparty faster than with existing solutions, without the disadvantages ofthe methods described above.

This need is addressed by the present invention in that it discloses amethod for use in a cellular access system in which there can be atleast one Radio Base Station which controls the traffic to and from acell in the system.

In the system in which the invention may be applied, there can be atleast one user with a User Equipment, UE, in the cell, and the RBS andthe UE can receive and transmit information between each other, so thatone of them is a sending party and the other one is a receiving party.

According to the method of the invention, errors in messages which arereceived can be detected by the receiving party by means of a firsterror detection method, and if errors are detected in received messages,the receiving party can request the transmitting party to retransmit amessage in which an error has been detected.

Further, according to the method of the invention, the receiving partykeeps a memory of the order in which it requests retransmissions, sothat erroneous retransmissions may be detected by the receiving partywithout the aid of the first error detection method. Upon detection ofan erroneous retransmission, the receiving party may request aretransmission from the transmitting party of the erroneousretransmission.

Thus, the invention provides a method for detecting transmission errorsof retransmitted blocks and/or NACK reports when the sequence number oferroneous radio blocks can not be decoded. Using the method of theinvention, there is no need for timers that are expensive to maintainand unreliable in the presence of scheduling delays.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail with reference to theappended drawings, in which

FIG. 1 shows an example of a system in which the invention may beapplied, and

FIGS. 2-4 show exemplary flow charts of the method of the invention, and

FIG. 5 shows a block diagram of an RBS of the invention, and

FIG. 6 shows a block diagram of a UE of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically shows a system 100 in which the invention may beapplied. The system 100 is a cellular wireless access system, and assuch comprises a number of cells, one of which is shown in FIG. 1 withthe reference number 110.

The cell 110 comprises at least one radio base station, an RBS, shown as120 in FIG. 1. The RBS 120 serves, inter alia, to control the traffic toand from users in the cell 110. The cell 110 can accommodate at leastone user terminal, with two user terminals being shown in FIG. 1, withthe reference numbers 130 and 140.

The system 100 is shown as a cellular telephony system, and theinvention will be described with reference to such a system, but itshould be pointed out that this is by way of example only, the inventionmay be applied to a number of different wireless access systems.

Also, the terminology used when describing the invention with referenceto the system 100 is merely intended to facilitate the reader'sunderstanding of the invention, and is not intended to restrict thescope of protection sought for the present invention. For example, theterm base station or radio base station, RBS, should be interpreted asmeaning a node in the system with the function of an RBS. In somesystems, for example, a function essentially corresponding to that ofthe RBS is performed by a node called Node B. Such systems are naturallyalso encompassed by the invention.

Similarly, it should be realized that the term user terminal or UE ismerely an example intended to facilitate the reader's understanding ofthe invention. For example, in some systems, the terms UT, UserTerminal, or MS, Mobile Station, are used. Naturally, such systems arealso encompassed by the scope of the present invention.

In addition, although the UEs 130, 140, are shown as cellular telephonesin FIG. 1, it should be realized that this is merely to facilitate theunderstanding of the invention, the UEs may be many other kinds ofdevices, portable or stationary, such as, for example, computers.

As explained previously in this text, the UEs 130, 140, can send trafficto the RBS 120, as well as being able to receive traffic from the RBS.If a receiving party, either a UE or an RBS detects an error in areceived message, or detects the absence of a message which has beenreceived, the receiving party can request the transmitting party (RBS orUE) to retransmit the damaged or missing message, by means of sending aso called NACK, Negative Acknowledge, message.

The present invention primarily aims at speeding up the datatransmission through fast detection of transmission errors onretransmitted data and NACK messages.

A basic idea of the invention is to detect errors of retransmissions orin NACK reports by means of letting the receiving party keep track ofthe sequence of requested retransmissions and compare it to the sequenceof received retransmissions. In other words, the order in whichretransmissions are requested by the receiving party is stored, and if,for example, the retransmission of block n was requested after theretransmission of block k, an error can be detected by the receivingparty if the retransmission of block n is received before theretransmission of block k is received, and a new NACK for block k can besent by the receiving party to the transmitting party.

Thus, according to the invention, errors of retransmissions or NACKreports are detected by the receiving party since that party keeps trackof or stores the sequence in which it requests retransmissions. If, forexample, the retransmission of block n was requested after theretransmission of block k, an error can be detected if block n isreceived before block k, and a renewed NACK for block k can be sent bythe receiving party to the transmitting party.

Some examples of how the method of the invention may be employed willnow be given in order to facilitate the reader's understanding of theinvention.

The examples will be illustrated by means of the flowcharts in FIGS. 2and 3.

Some abbreviations which will be used in the examples below are:

-   -   BSN—Block Sequence Number    -   ARQ—Automatic Repeat Request

In the examples below, it will, for the sake of simplicity, be assumedthat the RBS 120 is the receiving party and one of the UEs 130, 140, isthe transmitting party, but it should be pointed out that this is anexample only, either of the RBS/UE can be the transmitting or thesending party, the invention is equally applicable for either case.

First, with reference to the flow chart 200 in FIG. 2, the RBS 120detects that BSN 1 was received with an error, step 210, and a firstretransmission of BSN 1 is requested, step 220. How the first detectionof an error in received data is detected by the RBS is not within thescope of the invention, and can be done in a number of ways, such aserror detecting codes or detecting if the received BSN is “in sequence”or not.

In step 230, the reception of BSN 4 is also detected as erroneous, andin step 240 a first retransmission of BSN 4 is requested. In step 250,the first retransmission of BSN 1 is also received as erroneous, butwith a corrupt header, so the error is not detected by the first errordetection mechanism.

When the first retransmission of BSN 4 is received, step 260, the headeris correctly decoded, and since the RBS keeps track of when thisretransmission was requested, and thus knows that it was after therequest of retransmission of BSN 1, a retransmission out of sequenceevent is detected according to the invention, step 270, and a secondretransmission of BSN 1 is thus requested by the RBS in step 280.

Another example of an application of the invention is given in the flowchart 300 in FIG. 3: In step 310, the reception of BSN 1 from the UE isdetected as erroneous in the RBS, and a first retransmission of BSN 1 isrequested by the RBS in step 320. In step 330, the reception of BSN 4 isalso detected as erroneous, and a first retransmission of BSN 4 from theUE is requested by the RBS in step 335.

In step 340, the first retransmission of BSN 1 is also received aserroneous, and in step 350 a second retransmission from the UE isrequested by the RBS. In step 360, the first retransmission of BSN 4 isreceived with a corrupt header, so no error is detected by the firsterror detection mechanism.

However, when the second retransmission of BSN 1 is received, step 370,the RBS, by means of the invention, knows that this retransmission wasrequested after the retransmission of BSN 4, and a retransmission out ofsequence event is detected, step 380. In step 390, a new retransmissionof BSN 4 is thus requested from the transmitting party.

In a version of the invention, in order to allow the transmitting partyto use different priorities of the retransmissions without triggering aNACK from the receiving party by means of the present invention, theerror detection method of the invention could be modified so that beforea NACK for a block k is sent, i.e. before an error is indicated in thereception of block k, both a retransmission of block n should have beenreceived, and after that, any other block that is not a retransmissionshould have been received. This will be explained in more detail in thefollowing:

Using the principles of the invention explained with the aid of FIGS. 2and 3, the receiving party will detect errors in received dataincorrectly if the retransmissions are scheduled (usually by thetransmitting party) through some other priority algorithm than FIFO,First In, First Out.

If the scheduling priority of the retransmissions are unknown, but it isknown that the retransmissions are prioritized over new transmissions,the invention can be modified so that both a retransmission of block nwhich was requested after the retransmission of block k, is received,and after that, a first transmission of at least one other block that isnot a retransmission should also have been received. This version of theinvention is illustrated more closely in the flow chart 400 in FIG. 4:In step 410, the reception of a first transmission of BSN 1 is detectedas erroneous by means of the first error detection mechanism, and afirst retransmission is requested in step 415.

In step 420, the reception of BSN 4 is also detected erroneous, and afirst retransmission is requested, step 425. The first retransmission ofBSN 1 is also received erroneously, step 430, and a secondretransmission of BSN 1 is requested, step 435.

The transmitting party, for example, decides that the retransmission ofBSN 1 has higher priority than that of BSN 4, step 440, so thetransmitter resends BSN 1 first, step 445. When BSN 1 arrives at thereceiver, step 450, the receiving party detects that BSN 1 is receivedout-of-sequence, step 455, but it waits before it sends a NACK for BSN4. The second retransmission of BSN 4 is then received, step 460, butthe data and header are corrupt, so the receiver can not detect anerror. After that, the first transmission of BSN 9 is receivedcorrectly, step 465. The receiver has now received both a retransmissionout of sequence and after that a first transmission of at least oneother block, and can therefore detect an error, step 470, and sends aNACK for BSN 4, step 475.

FIG. 5 shows a rough outline of some of the components in an RBS 120 ofthe invention: As indicated in FIG. 5, the RBS 120 comprises means 121for receiving and transmitting information to and from a UE, so that theRBS 120 can be either the sending party or the receiving party withrespect to the UE.

The RBS 120 comprises first means 122 for detecting errors in messageswhich are received from a UE by means of a first error detection method.

The RBS 120 comprises means 123 for requesting a UE to retransmit amessage in which an error has been detected, and the RBS 120 alsocomprises means 124 for storing the order in which it requestsretransmissions, so that erroneous retransmissions may be detected byretransmission error detection means 125 without the aid of the firsterror detection means, following which the RBS 120 may request aretransmission from a UE of erroneously received retransmissions.

FIG. 6 shows a rough outline of some of the components in a UE 130 ofthe invention: As indicated in FIG. 6, the UE 130 comprises means 131for receiving and transmitting information to and from an RBS, so thatthe UE 130 can be either the sending party or the receiving party withrespect to the RBS.

The UE 130 comprises first means 132 for detecting errors in messageswhich are received from an RBS by means of a first error detectionmethod.

The UE 130 comprises means 133 for requesting an RBS to retransmit amessage in which an error has been detected, and the UE 130 alsocomprises means 134 for storing the order in which it requestsretransmissions, so that erroneous retransmissions may be detected byretransmission error detection means 135 without the aid of the firsterror detection means, following which the UE 130 may request aretransmission from an RBS of erroneously received retransmissions.

The invention is not limited to the examples of embodiments describedabove and shown in the drawings, but may be freely varied within thescope of the appended claims.

It should also be pointed out that in order to facilitate for thereader, the term “message” has been used more or less consistentlythroughout this text to describe traffic between two parties. As such,the word “message” in this text can denote both such traffic whichsometimes is referred to as “data” as well as traffic which istraditionally referred to as “messages”.

The invention claimed is:
 1. A method, performed at a receiving device,for detecting erroneous retransmissions of messages, the methodcomprising: detecting errors in messages received from a sending device,wherein the messages include retransmitted messages; requesting thesending device to retransmit the messages in which errors are detected,and recording a retransmission order in which retransmissions of themessages are requested, wherein the retransmission order is updated toadd entries corresponding to messages whose retransmissions are detectedto be erroneous and messages whose transmissions are detected to beerroneous; and detecting erroneous retransmissions of messages based onwhether the retransmitted messages are received from the sending devicein the recorded retransmission order and without aid of a retransmissiontimer.
 2. The method of claim 1 wherein the specified retransmissionorder comprises a non-sequential order of messages in which the errorswere detected.
 3. The method of claim 1 further comprising storing thespecified retransmission order at the receiving device.
 4. The method ofclaim 1 wherein detecting erroneous retransmissions of messages based onwhether the retransmitted messages are received from the sending devicein the specified retransmission order and without aid of aretransmission timer comprises detecting a retransmitted message asbeing erroneous if the retransmitted message is not received in thespecified retransmission order.
 5. The method of claim 1 furthercomprising detecting the erroneous retransmissions of retransmittedmessages in accordance with a priority assigned to the retransmittedmessages by the sending device.
 6. The method of claim 5 wherein thespecified retransmission order indicates that the sending device is toretransmit a second message before the sending device retransmits afirst message, and wherein detecting the erroneous retransmissions ofretransmitted messages in accordance with the assigned prioritycomprises: receiving the retransmissions of the first and secondmessages from the sending device wherein the first message has beenassigned a priority that is higher than a priority of the secondmessage; receiving a third message from the sending device, wherein thethird message is not a retransmitted message and has been assigned apriority by the sending device that is lower than the first and secondmessages; and detecting an erroneous retransmission of the secondmessage if the first message is not received in the specifiedretransmission order, and if the third message is received before thesecond retransmitted message is correctly received.
 7. The method ofclaim 6 further comprising delaying sending a retransmission request tothe sending device for the first message until determining whether thesecond message was erroneously received.
 8. The method of claim 1wherein requesting the sending device to retransmit the messages inwhich errors are detected in a specified retransmission order comprisesrequesting a retransmission of a message each time an error is detected.9. A receiving device for detecting erroneous retransmissions ofmessages transmitted by a sending device via a communications network,the receiving device configured to: detect errors in the messagesreceived from the sending device, wherein the messages includeretransmitted messages; request the sending device to retransmit themessages in which errors are detected, and recording a retransmissionorder in which retransmissions of the messages are requested, whereinthe retransmission order is updated to add entries corresponding tomessages whose retransmissions are detected to be erroneous and messageswhose transmissions are detected to be erroneous; and detect erroneousretransmissions of messages based on whether the retransmitted messagesare received from the sending device in the recorded retransmissionorder and without aid of a retransmission timer.
 10. The receivingdevice of claim 9 wherein the specified retransmission order comprises anon-sequential order of messages in which the errors were detected. 11.The receiving device of claim 9 wherein the receiving device is furtherconfigured to store the specified retransmission order.
 12. Thereceiving device of claim 9 wherein the receiving device is furtherconfigured to detect a retransmitted message as being an erroneouslyretransmitted message if the retransmitted message is not received inthe specified retransmission order.
 13. The receiving device of claim 9wherein the receiving device is further configured to detect theerroneous retransmissions of retransmitted messages in accordance with apriority assigned to the retransmitted messages by the sending device.14. The receiving device of claim 13 wherein the specifiedretransmission order indicates that the sending device is to retransmita second message before the sending device retransmits a first message,and wherein to detect the erroneous retransmissions of retransmittedmessages in accordance with the assigned priority, the receiving deviceis further configured to: receive the retransmissions of the first andsecond messages from the sending device wherein the first message hasbeen assigned a priority that is higher than a priority of the secondmessage; receive a third message from the sending device, wherein thethird message is not a retransmitted message and has been assigned apriority by the sending device that is lower than the first and secondmessages; and detect an erroneous retransmission of the second messageif the first message is not received in the specified retransmissionorder, and if the third message is received before the secondretransmitted message is correctly received.
 15. The receiving device ofclaim 14 wherein the receiving device is further configured to send aretransmission request to the sending device for the first message untilit is determined whether the second message was erroneously received.16. The receiving device of claim 9 wherein the receiving device isconfigured to request a retransmission of a message each time an erroris detected.
 17. The receiving device of claim 9 wherein the receivingdevice comprises one of a radio base station (RBS) communicating with aUser Equipment (UE), and wherein the sending device comprises the otherof the RBS and the UE.
 18. A method, performed at a receiving device,for detecting erroneous retransmissions of messages, the methodcomprising: detecting errors in messages received from a sending device,wherein the messages include retransmitted messages; requesting thesending device to retransmit the messages in which errors are detected,and recording a retransmission order in which retransmissions of themessages are requested, wherein the retransmission order is updated toadd entries corresponding to messages whose retransmissions are detectedto be erroneous and messages whose transmissions are detected to beerroneous; and detecting erroneous retransmissions of messages if theretransmitted messages are not received in the recorded retransmissionorder.
 19. A Radio Base Station (RBS) for use in a cellular accesssystem, for controlling of traffic to and from a cell in the system, inwhich cell there can be at least one User Equipment (UE) in the cell,the RBS configured to: detect errors in messages received from the UE,wherein the messages include retransmitted messages; request the UE toretransmit the messages in which errors are detected, and recording aretransmission order in which retransmissions of the messages arerequested, wherein the retransmission order is updated to add entriescorresponding to messages whose retransmissions are detected to beerroneous and messages whose transmissions are detected to be erroneous;and detect erroneous retransmissions of messages if the retransmittedmessages are not received in the recorded retransmission order.
 20. AUser Equipment (UE) for use in a cellular access system that includes aRadio Base Station (RBS) for the control of traffic to and from a cellin the system in which cell the UE can be used, the UE is configured to:detect errors in messages received from the RBS, wherein the messagesinclude retransmitted messages; request the RBS to retransmit themessages in which errors are detected, and record a retransmission orderin which retransmissions of the messages are requested, wherein theretransmission order is updated to add entries corresponding to messageswhose retransmissions are detected to be erroneous and messages whosetransmissions are detected to be erroneous; and detect erroneousretransmissions of messages if the retransmitted messages are notreceived in the recorded retransmission order.